1. Field of the Invention
The present invention is generally related to a method and apparatus for identifying a plug-to-plug short (short circuit) from a charged particle microscopic image, and more particularly to a method and apparatus for identifying a plug-to-plug short as an abnormal voltage contrast (VC).
2. Description of the Prior Art
During charged particle beam inspection of a sample of a semiconductor device, a charged particle microscopic image is formed from secondary charged particles released by the sample while it is bombarded with a primary charged particle beam. In the obtained image, patterns on the sample are typically displayed in grey levels. By analyzing this grey level image of the sample, the presence of defects, i.e., abnormal physical and electrical characteristics of the sample, can be represented in the form of voltage contrast (VC).
In the manufacture of a semiconductor device, narrow gaps between conductive metal lines often give rise to voids or keyholes during the dielectric deposition process. When this happens, metal filler such as titanium (Ti), titanium nitride (TiN) and tungsten (W) deposits into the keyholes and causes shorts (short circuits) between neighboring metal plugs. This yield-impacting defect is commonly called a plug-to-plug short defect. In some cases, it is called the “piping” defect, as well. The piping plugs are generally found between two parallel lines underneath the surface of the sample. FIG. 1 illustrates a plug-to-plug short defect, as represented schematically in the context of a top view image 100 of a semiconductor device comprising plugs 101 and metal lines 102. Reference number designators 110 and 120 identify cross-sectional views along line A-A′ and line B-B′ of image 100, respectively. A plug-to-plug short defect 103 causes an undesired connection between neighboring plugs 101. As shown, plug-to-plug short defect 103 can involve two or three plugs 101.
Currently, to inspect for the presence of this defect in real time, in-line monitoring with a charged particle scanning microscope is typically employed. The imaging process is typically performed by repeatedly line-scanning a charged particle beam over the sample with a line-to-line advancement direction substantially perpendicular to the line-scan direction. This scan mode is generally referred to as the raster scan. As a result, a two-dimensional (2D) array of scan lines is formed on the sample, and a grey level image of the sample is thus obtained therefrom.
For a long “string” type of plug short caused by long keyhole, long string bright voltage contrast (BVC) between tungsten (W) plugs has been observed, typically standing out from the environmental darker voltage contrast (DVC) pixels.
However, when inspecting piping between neighboring plugs in a grey level image, especially when the defective plugs are aligned on an axis parallel to the line-to-line advancement direction, the defective plugs often appear as one slight BVC plug pattern neighbored by one or more normal, slightly dark, or less bright DVC plug patterns. FIG. 2 is a schematic illustration of a plug-to-plug short defect and leakage defect in a grey level image. With reference to the top half of this figure, a plug-to-plug short defect 201 is illustrated in image 200(a), and a leakage defect 202 is illustrated in image 200(b). As shown in this example, plug-to-plug short defect 201 displays successive plug patterns with the left one in BVC and the right one in DVC, while leakage defect 202 displays a single BVC. It can be seen from images 200(a) and 200(b) that the plug-to-plug short defect 201 is not distinct from the leakage defect 202 because in both cases only one single BVC is clearly observed. The line-scan direction and line-to-line advancement direction for forming images 200(a) and 200(b) are illustrated by arrows 210 and 212 (which, in this example, is from left to right), respectively.
This is mainly because when the first defective plug of the plug-to-plug short defect 201 along the line-to-line advancement direction 212 (which is the left plug pattern) is encountered and scanned, most free electrons in the other defective plug (which is electrically connected to the scanned plug through piping) are attracted by the accumulated positive charging on the surface of the scanned plug and, thus, move to the scanned plug. As a result, the accumulated charging on the scanned plug is neutralized, causing it to display BVC in the obtained grey level image. When the next plug (which is the right plug pattern) is scanned, as most of its free electrons are gone by the moment of scanning, positive charging will accumulate on its surface. Therefore, this latter scanned plug displays normal (darker) VC or DVC in the obtained grey level image. This makes it difficult to identify (distinguish) these defective plug patterns as consecutive BVCs, as compared to single BVCs resulting from, for example, a leakage defect.
The same confusion may appear in a three-plug piping situation, as well. With reference to the bottom half of FIG. 2, image 200(c) illustrates such a case where, again, the three defective piping plug patterns 201 are aligned with each other and an axis parallel to the line-to-line advancement direction 212 (which, in this example, is from right to left). In such a case, only the first defective plug pattern (which, this time, is the one on the right) in the line-to-line advancement direction 212 of scan can be easily observed as a BVC in the grey level image. The other two plug patterns may just look normal (darker VC). Therefore, by comparison with image 200(d) which illustrates a leakage defect 202 displaying a single BVC, it can be seen that the plug-to-plug short defect 201 is still difficult to be distinguished (e.g., not easily or readily distinguishable) from the leakage defect 202.
Accordingly, a method for improving the contrast between these two types of defects in a grey level image is desired.